1. Field of the Invention
The present invention relates to high-voltage insulators made from ceramic materials having a shank and ends where the ends are at least 1.05 times as thick as the shank. Caps are shrink-fit around the ends of the insulators to provide a tight seal.
2. Description of Related Art
High voltage insulators of ceramic materials are mainly used in outdoor switching stations and outdoor lines. They comprise an elongated insulation body which is equipped with shields for the formation of a leakage path which is matched, to the atmospheric conditions. The shields are moulded on the insulator shank whose thickness is determined by the mechanical requirements. At the ends of the insulation body or the insulator shank there are located metal caps via which the force transmission from the insulator shank to components leading further takes place. High voltage insulators are usually configured so as to have rotational symmetry, if the asymmetry of the caps, for example, as a result of individual links is ignored; the insulator caps concentrically surround the ends of the insulator shank. The mechanical loadability is determined not only by the shank diameter of the insulator, but also by the configuration of the shank ends, the manner in which the metal caps are fixed to the shank and the configuration and the material of the metal caps and also the type of mechanical stresses, which can, in principle, be tensile forces, compressive forces, flexurel forces and torsional forces or combinations of these forces. The constructions of the metal caps therefore depend on the type of stress prevailing in the particular case.
In the case of the known high voltage insulators, solid or hollow, the metal caps are slipped onto the insulator end to be reinforced and the gap between the insulator shank and the metal cap is filled with a setting filler material, such as various types of cement, lead or casting resin. The ends of the insulator body are here configured differently. Thus, the ends of tensile-stressed series path stabilizers (suspended insulators) have a conical configuration and are glazed and are frequently fixed in the metal cap by means of cast lead. In the case of post insulators subjected to flexural and/or torsional stresses, the insulation bodies are usually provided with cylindrical ends. The ends can here be made rough in various ways, e.g. fluted, spread with grit or corrugated. Portland cement is mainly used as filler material. The flexural strength of post insulators is strongly dependent on the ratio of filler depth to insulator shank diameter. Metal caps for suspended and post insulators usually comprise galvanized cast iron, because in the case of these insulators no great accuracy is required for the external dimensions. Where high demands are placed on the accuracy of the external dimensions of the insulators, the metal caps usually comprise aluminum alloys which have to be very accurately machined and require no additional corrosion protection after machining. To achieve the necessary precision of the insulator dimensions during cementing of the caps, efforts have to be made to relieve stresses in the positioning of the caps.
DE-A-36 43 651 discloses the shrink-fitting of the metal caps onto the ends of spherical-headed ceramic insulators. According to this method, the components are heated together, joined and cooled together, so that the ceramic workpiece is not damaged. This type of joining technique is very complicated for insulators, since hollow insulators in particular can have dimensions in the meter range. The invention is to provide a solution here.